Lithium is a soft, silver-white metal belonging to the alkali metal group of chemical elements. Lithium metal is a high energy density battery anode material due to its high theoretical specific capacity (3860 mAh/g), low density (0.59 g/cm3), and low negative reduction potential (−3.040 V vs. SHE). Comparatively, a graphite anode used in lithium ion batteries has a specific capacity of about 350 mAh/g. Utilizing lithium metal anodes can offer a 10× increase in capacity. Lithium is present in over fifty compounds and has two stable isotopes, Lithium-6 and Lithium-7. It is the lightest metal and the least dense solid element. Lithium is highly reactive and flammable, though it is the least reactive of the alkali metals. Since lithium only has a single valence electron that is easily given up to form a cation, it is a good conductor of heat and electricity.
Because of its high reactivity, lithium does not occur freely in nature. Instead, lithium only appears naturally in compositions, usually ionic in nature, such as lithium carbonate. Therefore, lithium metal can be obtained only by extraction of lithium from compounds containing lithium.
The two most common ways of obtaining lithium are currently through extraction of lithium present in either spodumene or brine, producing carbonate. Lithium is then obtained from the lithium carbonate in two phases: (1) conversion of lithium carbonate into lithium chloride, and (2) electrolysis of lithium chloride using a high-temperature molten salt such as LiCl.
To convert lithium carbonate to lithium chloride, the lithium carbonate is heated and mixed with hydrochloric acid (typically 31% HCl) in an agitated reactor to generate lithium chloride, carbon dioxide and water as shown below:Li2CO3(s)+2HCl(aq)→2LiCl(aq)+H2O(aq)+CO2(g)
The formed carbon dioxide is vented from the reactant solution. A small amount of barium chloride can be added to precipitate any sulfate. After filtering, the solution is evaporated to a saleable 40% LiCl liquid product. Potassium chloride can be added to provide a dry lithium chloride-potassium chloride (45% LiCl; 55% KCl) of decreased melting point (614° C. to approximately 420° C.). Then the lithium chloride-potassium chloride (45% LiCl; 55% KCl) in a molten pure and dry state can be utilized to produce lithium metal in a steel reaction cell using electrolysis as shown in the reactions below:Cathode: Li++e−→Li metalAnode: Cl−→½Cl2+e−Total: 2LiCl→2Li+Cl2 
A conventional steel reaction cell has an exterior ceramic insulation and a steel rod on the bottom as a cathode. The anode is constructed of graphite, which slowly sloughs-off during processing. When the cell is heated, lithium metal accumulates at the surface of the cell wall and is then poured into ingots. Chlorine gas generated by the reaction is routed away. Typically, the electrolysis process is operated with a cell voltage from 6.7 V to 7.5 V, and the typical cell current is in the range of about 30 kA to 60 kA. The electrolytic processing consumes about 30 kWh to 35 kWh of electricity energy and about 6.2 to about 6.4 kg LiCl to produce one kilogram lithium metal with about 20% to 40% energy efficiency. Improvements to the molten salt electrolysis process have involved the selection of different types of electrolytic molten salts that allow for a decrease in operating temperatures.
For example, U.S. Pat. No. 4,156,635 to Cooper et al. describes an electrolytic process for the production of lithium using a lithium-amalgam electrode. The lithium is recovered from its molten amalgam using a fused-salt molten electrolyte consisting of a mixture of at least two alkali metal halides. The metal halides may include lithium iodide, lithium chloride, potassium iodide, and potassium chloride. U.S. Pat. No. 4,156,635 teaches that the lithium amalgam is produced by electrolysis of an aqueous solution of a lithium salt such as lithium hydroxide in the present of a mercury cathode. The lithium amalgam is then circulated between an aqueous cell containing the lithium salt solution and a fused-salt cell containing the molten electrolyte, and the lithium amalgam serves as a bipolar electrode.
Another low temperature technology involves electrolysis of brine to form chlorine at an anode and sodium hydroxide or potassium hydroxide via a series of cathode reactions. The formation of either of these hydroxides can involve the reduction of Li+ to metal at a liquid mercury cathode, followed by reaction of the formed mercury amalgam with water. The process operates near room temperature with a lower voltage than required for the molten salt system.
U.S. Pat. No. 8,715,482 to Amendola et al. provides a system and process for producing lithium without a mercury electrode. The liquid metal alloy electrode system of U.S. Pat. No. 8,715,482 includes: an electrolytic cell comprising a liquid metal cathode and an aqueous solution wherein the aqueous solution containing lithium ion and at least an anion selected from sulfate, trifluoromethane sulfonate, fluorosulfonate, trifluoroborate, trifluoroacetate, trifluorosilicate and kinetically hindered acid anions, and wherein the lithium ion is produced from lithium carbonate. A heating system maintains temperature of the cell and liquid metal circulating systems higher than the melting point of the liquid metal cathode but lower than the boiling point of the aqueous solution. The reduced lithium from the electrolytic cell is extracted from the liquid metal cathode using a suitable extraction solution and a distillation system for isolating the lithium metal. This system is solid at room temperature and is less toxic than previous systems.
U.S. Pat. No. 6,770,187 to Putter et al. discloses another process that overcomes some of the high energy consumption and high temperature requirements of prior art processes. The process enables recycling of alkali metals from aqueous alkali metal waste, in particular, lithium from aqueous lithium waste. U.S. Pat. No. 6,770,187 provides an electrolytic cell comprising: an anode compartment which comprises an aqueous solution of at least one alkali metal salt, a cathode compartment, and an ion conducting solid composite that separates the anode compartment and the cathode compartment from one another, wherein that part of the surface of the solid electrolyte composite that is in contact with the anode compartment and/or that part of the surface of the solid electrolyte that is in contact with the cathode compartment has/have at least one further ion-conducting layer. The electrolyte used in U.S. Pat. No. 6,770,187 is water or water with organic solvent.
Lithium metal readily reacts with water to form hydrogen gas and lithium hydroxide. Because of its reactivity, lithium metal, once extracted from a lithium compound, is usually stored under cover of a hydrocarbon, often petroleum jelly. Though the heavier alkali metals can be stored in more dense substances, such as mineral oil, lithium metal is not dense enough to be fully submerged in these liquids. In moist air, lithium metal rapidly tarnishes to form a black coating of lithium hydroxide (LiOH and LiOH⋅H2O), lithium nitride (LiN) and lithium carbonate (Li2CO3).
Because of lithium's high electrochemical potential, it is an important component of electrolytes and electrodes in batteries. For example, lithium metal is commonly used as an anode material in a lithium primary battery. Lithium metal is currently used as an anode material in three commercially available rechargeable batteries: lithium sulfur batteries developed by Sion Power Company and Oxis Energy (UK), and a lithium metal polymer battery used by the Bolloré Group. However, the lithium sulfur battery developed by Sion Power, mentioned above, requires a protective cover on the anode, and the lithium metal polymer battery technology utilizes a low capacity cell system specifically integrated into an electric vehicle. Mass adaptation of lithium metal anodes in rechargeable batteries has not yet been realized due to problems with dendrite formation and interfacial reactions. On the other hand, the lithium sulfur battery developed by Oxis Energy has a very poor cycle life of about 60 cycles.
Lithium metal anodes are particularly desirable for use in batteries, since batteries using a lithium metal anode have much higher energy densities than batteries using graphite or other conventional non-lithium anode materials. Lithium metal anodes have the highest specific capacity value of 3,860 mAh/g and depending on the system can achieve energy densities of 1470 Wh/Kg and above.
However, lithium metal produced by conventional lithium producing processes contains impurities that undesirably cause dendrite formation when the lithium metal is used as an anode in a rechargeable battery. The conventional process produces lithium foils via extrusion, with surface defects and cracks which serve as nucleation sites for dendrite growth For example, during charge and discharge cycling of the battery, impurities in the lithium metal cause side reactions and form lithium crystals (i.e., “dendrites”) to emerge from the surface of the anode and spread across the electrolyte. In lithium polymer systems, dendrites begin forming at the electrode underneath the polymer/electrode interface prior to coming into contact with the electrolyte. In the same system, dendrites have also been observed on the uncycled lithium anode. Dendrite formation causes the battery to short-circuit, thereby increasing the temperature of the battery to potentially unsafe levels, resulting in thermal runaway or even death. Therefore, higher-purity lithium metal with a stable, uniform solid electrolyte interphase (SEI) layer allowing for fast electron transfer that does not contain the impurities resulting from conventional lithium producing processes is desirable, and would result in a two-fold reduction of dendrite suppressing efforts, firstly by eliminating nucleation sites, and secondly by eliminating side reactions due to impurities.
Metallic lithium can also be used as a flux for welding or soldering to promote fusing of metals to eliminate oxide formation by absorbing impurities. Its fusing quality is important as a flux for producing ceramics, enamels and glass. Metallic lithium is also used in the metallurgical industry to form alloys containing lithium. High purity lithium metal is desirable in forming alloys to reduce the overall impurity level in the alloys. High purity lithium metal and alloys containing such high purity metal are also desirable as an improvement in currently available primary lithium batteries. Furthermore, increasing the concentration of lithium metal in an alloy, such as a lithium aluminum alloy, would result in an increased performance and lifespan for primary lithium batteries.
Lithium metal may also be used to make certain lithium compounds. For example, lithium fluoride is a common additive in battery electrolytes and has been shown to form stable SEI layers when lithium is plated. On the other hand, lithium oxide may be used as a flux for processing silica to glazes of low coefficients of thermal expansion, lithium carbonate (Li2CO3) may be used as a component in ovenware, and lithium hydroxide (LiOH) may be used as a strong base that can be heated with a fat to produce a lithium stearate soap. Lithium hydroxide monohydrate may be used as feedstock to produce a cathode material used in cylindrical cells, such as the Panasonic 18650 rechargeable lithium-ion battery. Notably, nickel cobalt aluminum (NCA) cathodes used in the cylindrical cells may require a high-purity LiOH. The cylindrical cells may be used in a battery pack. Lithium soap can be used to thicken oils and in the manufacture of lubricating greases. Lithium metal may also be used to form lithium fluoride. Lithium compounds are used in a wide variety of lithium battery electrolytes, for example LiPF6, a common Lithium-ion battery electrolyte. Lithium salts such as lithium carbonate, lithium citrate and lithium orotate are also used in the pharmaceutical industry as mood stabilizers to treat psychiatric disorders such as depression and bipolar disorder. High purity lithium is desirable when making these lithium compounds in order to reduce the overall impurity level in the resulting compounds.